This invention relates generally to filters, and more particularly, to low frequency filters employing the thermal properties of electrical devices to provide the filtering function.
Low frequency high pass, low pass, band pass and band-elimination filters are well known. Such filter generally employ large value resistors, inductors or capaitors to achieve the low frequency filtering function. Other such devices utilize the capacity multiplying effects of a Miller integrator to synthesize an apparent large value capacitor for a low frequency filter. Still other circuits employ active components to provide the filtering function and utilize feedback techniques or digital sampling techniques. Attempts at synthesizing electro-thermal low pass filters have also been made. One such attempt is described in a doctoral dissertation entitle "Electro-Thermal Integrated Circuits" submitted by Paul Russel Gray to the University of Arizona in 1969.
Whereas these techniques provide a way to achieve low frequency filtering, the use of high value resistors, capacitors, and inductors results in bulky and expensive structures that are not particularly suitable for modern miniaturized equipment. The use of Miller integrators results in a filter having a limited quality factor and dynamic range. Analog active filters tend to be rather complex and oscillatory, and generally must rely on precision components to provide the filtering function. Digital active filters also tend to be complex and require relatively complex clock circuitry to make them operate. Previous attempts to fabricate electro-thermal filters have not been entirely satisfactory because of the complexity of the mathematical formulas involved, and the undesirability of the multiple pole transfer functions that resulted from such previous attempts at filter synthesis. Furthermore, the electrical characteristics of the prior art filters tend to be dependent on the ambient temperature.
Accordingly, it is an object of the present invention to provide a new and improved low frequency filter.
It is another object of the present invention to provide an improved filter that provides low frequency filtering without the use of inductors or capacitors as frequency determining elements.
Yet another object of the present invention is to provide a new and improved low frequency filter that is relatively simple and inexpensive to produce.
A further object of the present invention is to provide a readily realizable electro-thermal filter having only a single predominant pole in its transfer function in the frequency range of interest.
Another object of the present invention is to provide an electro-thermal filter having a transfer function that is relatively independent of the ambient temperature.
Another object of the present invention is to provide a low frequency filter that is compatible with miniaturized electronic equipment and which can readily be fabricated using semiconductor techniques.
In accordance with a preferred embodiment of the invention, the thermal properties of transistor amplifiers are used to provide the filtering function. A first transistor amplifier, generally a differential amplifier, is fabricated on a semiconductor chip. The first amplifier is used differentially to heat the second amplifier in response to an input signal applied to the first amplifier. The second amplifier is thermally coupled to the first amplifer, preferably by fabricating portions of the two amplifiers on the same semiconductor chip and bonding the chip to an insulating substrate. The heat applied to the second amplifier changes one of its operating characteristics, such as the current balance between transistors, in response to the heat generated by the first amplifier. The thermal transfer function of the thermal coupling between amplifiers determines the poles and zeros of the electrical transfer function of the filter. A feedback network is provided around the second amplifier to stabilize the operating point of the second amplifier, the feedback loop thereby providing a signal representative of the input signal filtered by the action of the thermal coupling.
The use of differential amplifiers as heating and sensing elements causes common mode signals, such as those occurring as a result of changes in the ambient temperature, to be rejected. The size, shape and position of the heater and sensor transistors is selected to cancel various poles in the transfer function such that a single pole response is approximated. In addition, the size, shape and position of the heater and sensor transistors may be adjusted to determine whether the thermal characteristics of the semiconductor chip or those of the substrate primarily determine the filter transfer function. This provides increased design flexibility and the ability to select the frequency range of the filter by utilizing either the thermal characteristics of the semiconductor chip, those of the substrate, or a combination of both to determine the dominant pole.